For practical reasons it is often desirable to convert a laser light into a different wavelength. For example, in optical recording technology where semiconductor diode lasers are used as light sources, converting the near-infrared laser light into a blue light can significantly increase optical recording density. Optical wavelength conversion is usually accomplished by second-order nonlinear optical processes such as second-harmonic generation, sum- or difference-frequency generation, or other parametric processes.
Through these processes, one or two laser beams interact with a nonlinear optical medium to generate a coherent light beam at a different wavelength. The efficiency of the nonlinear wavelength conversion depends on the intensity of the excitation laser beam(s). If relatively low-power lasers (such as the diode lasers) are used, instead of performing the wavelength conversion in a bulk nonlinear optical medium, it is more advantageous to achieve it in a waveguide made of the nonlinear optical medium, the latter confining the laser beam(s) in a small area over the entire length of the guide and thereby yielding a much higher wavelength conversion efficiency.
In waveguides as well as in bulk media, normally two conditions must be met to achieve efficient wavelength conversion. First, the phase velocities of the interacting light waves must be matched--so called "phase-matching" condition. Second, the transverse field profiles of the light waves involved must overlap well with one another. To date, various schemes for phase-matched wavelength conversion in nonlinear optical waveguides have been proposed or experimentally demonstrated, either utilizing waveguide modal dispersion or periodic structures that modulate linear or nonlinear optical properties. Achieving phase matching using periodic structures has the advantage that, unlike matching waveguide modal dispersion, it does not impose restrictions on the dimensions of the waveguides, nor on any particular waveguide-mode combinations involved in the nonlinear processes.
The structure of a periodic waveguide for phase-matched nonlinear wavelength conversion is schematically shown in FIG. 1, where the wave-guiding region is made of a nonlinear optical medium, while the cover and the substrate regions may be either nonlinear optically active or passive. The cover and the substrate regions may generically be called cladding regions, i.e., regions which surround the waveguide, but which do not act as waveguides per se. Within the waveguide, the linear or nonlinear (or both) optical properties are spatially modulated along the wave propagating direction. The phase matching is achieved by making the modulation period exactly compensate for the mismatch among the phase velocities of the interacting light waves. [See, for example, A. Yariv and M. Nakamura, "Periodic structures for integrated optics", IEEE J. Quantum Electron. QE-13, 233 (1977).]Usually, under the phase-matching condition, higher efficiency for nonlinear wavelength conversion is obtained in periodic waveguides with the nonlinear (rather than linear) optical properties modulated. For example, Khanarian et al. U.S. Pat. Nos. 4,971,416 and 4,865,406 disclose a waveguide construction that consists essentially of a wave-guiding region made of a periodically poled nonlinear optical polymer film and two cladding, i.e., non-guiding, regions made of optically passive polymer films.
However, there are practical difficulties in fabricating many nonlinear optical materials into such a periodic waveguide. For example, with the Langmuir-Blodgett technique, thin films of certain organic nonlinear optical materials can be prepared, but their (linear or nonlinear) optical quality tends to degrade with increasing film thickness. [See, for example, the review article by S. Allen, "Langmuir-Blodgett films for nonlinear optical applications", in Materials for Nonlinear and EIectro-optics 1989, Inst. Phys. Conf. Ser. No. 103 (Institute of Physics, Bristol and New York, 1989), p. 163.]. Consequently, it is usually difficult to make the nonlinear optical Langmuir-Blodgett films into waveguides (typically a few .mu.m's in the transverse dimensions) of desirable optical properties.
Second-harmonic generation from a nonlinear optically active monomolecular layer (monolayer) has been incorporated in an otherwise optically passive waveguide. H. A. Haus et al., Appl. Optics, 26, 4576 (1987), proposed a waveguide structure, shown in FIG. 2a that consists of a passive waveguide, having a refractive index periodically modified along the guide, and a nonlinear optically active monolayer located at the boundary between the guide and the cover media. The monolayer can be viewed as a nonlinear optical source where the second-harmonic generation process actually occurs, and the passive waveguide allows confined propagation of both the input fundamental wave and the generated second-harmonic wave. Here, phase matching is achieved by adjusting the period of the refractive-index modulation.
G. A. Reider et al., Optics Commu., 68, 149 (1988), proposed a waveguide structure, depicted in FIG. 2b, that consists of essentially the same components as in FIG. 2a--but instead of a periodic guide region, the monolayer is spatially modified into a periodic grating-like structure for allowing phase-matched second-harmonic generation in the composite waveguide.
Recently, A. Bratz et al., Appl. Phys. B, 50, 393 (1990), have proposed a modified waveguide structure, shown in FIG. 2c, where the nonlinear optically active monomolecular layer is situated in the middle of the passive guide region. Compared to the waveguides in FIGS. 2a and 2b, this waveguide is predicted to yield a higher second-harmonic generation efficiency. Considering a model waveguide that contains a dye monolayer with a second-harmonic generation susceptibility of 10.sup.-13 esu (per monolayer), and ignoring optical absorption of the second-harmonic wave by the monolayer, Bratz et al. have predicted a second-harmonic generation conversion efficiency of 0.001% for a 1 cm-long waveguide and a 100 mW input power.
From a practical viewpoint, however, known compounds having a second-harmonic generation susceptibility of 10.sup.-13 esu (or larger) per monolayer often exhibit high optical absorptivity at the second-harmonic wavelength. For example, the value of the monolayer second-harmonic generation susceptibility used by Bratz et al. is based on that of a hemicyanine dye, but in that case the second-harmonic wavelength is at the peak of the dye's optical absorption band [G. Marowsky et al., Chem. Phys. Lett., 147, 420 (1988)]. The effect of the optical absorption by the monolayer is to reduce the second-harmonic generation conversion efficiency predicted by Bratz et al. by two or more orders of magnitude (i.e., an output less than 10.sup.-5 % or 0.01 .mu.W). This optical power level is to be compared, for example, with a power level of about 50 .mu.W required for reading an optical disk.
Except for the proposal by Haus et al. (where a method for fabricating the periodic passive waveguide was suggested), the other two proposals dealt only with theoretical models without suggesting practical procedures for fabricating the composite waveguides.
It is therefore an object of the present invention to provide a waveguide for efficient second-order nonlinear wavelength conversion that includes both second-harmonic generation and sum-frequency and difference-frequency generation.
It is a further object of the present invention to provide a waveguide having improved conversion efficiency.
It is a further object of the present invention to provide a waveguide comprising a nonlinear optically active film, in which the net second order nonlinearity (i.e., the total nonlinearity of the film) increases with the film thickness, embedded in an otherwise optically passive waveguiding medium.
It is a further object of the present invention to provide a fabrication process for preparation of such a waveguide.